This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Ming explored the use of several different K emission X-ray sources (W, Mo, Rh) to improve the contrast-to-noise for small animal digital subtraction angiography. Use of a K emitter will improve contrast to noise by: 1) flux production of the K emitters is much more efficient at low energies than Bremstrhalung;2) the quantum detection efficiency of the detector phosphors is much higher at lower energies (detecting phosphors must be thin to limit resolution degradation from light scatter);and 3) photoelectric absorption differences between soft tissue, calcification and iodine/gadolinium (potential vascular contrast agents) increase at lower energy. Based on simulation results, Ming enhanced our current micro-x-ray system for small animal digital subtraction angiography.